The present invention relates to a semiconductor device and a method for manufacturing a semiconductor device, and more particularly, to a semiconductor including an element isolating region and a method for manufacturing such a semiconductor device.
In a semiconductor device of the prior art, an element such as a bipolar transistor is formed by performing an element isolation process. Such a bipolar transistor is formed on a substrate with an embedment layer having a high impurity concentration. Known element isolation processes include isolation of elements with a field oxidation film, which is formed by performing local oxidation of silicon (LOCOS), and formation of a deep trench to isolate a high concentration impurity layer in a substrate. When a field oxidation film is formed by performing LOCOS, in addition to the flatness of the surface of an element isolating region decreasing, the area of the element isolating region is increased due to a “bird's beak” that occurs during the formation of the field oxidation film. This makes it difficult to further miniaturize such a semiconductor device.
Accordingly, Japanese Patent No. 3382063 proposes shallow trench isolation (STI) that enables the formation of a flat and miniaturized element isolating region.
FIG. 14 is a cross-sectional view showing a semiconductor device including an element isolating region formed by performing STI in the prior art. In FIG. 14, an N+ embedment layer 102 and an N-type epitaxial silicon layer 103 are formed on a silicon substrate 101. An STI trench is formed in the N-type epitaxial silicon layer 103. An insulative film 104 is formed on the inner surface of the STI trench. The STI trench is filled with an embedment film 105. A deep trench formed in the STI trench has a depth reaching the silicon substrate 101. An insulative film 106 is formed on the inner surface of the STI trench. The deep trench is further filled with an embedment film 107.
In the STI technique of the prior art, each corner in the STI trench and the deep trench (opening edge 200a of the STI trench, bottom edge 200b of the STI trench, boundary edge 200c between the STI trench and the deep trench, and bottom edge 200d of the deep trench) is rounded. Further, the insulative films 104 and 106 have about the same thicknesses.
The rounded corners (200a, 200b, 200c, and 200d) of the STI trench and the deep trench prevent the concentration of stress, which results from the shape of the element isolation structure, from causing defects. However, when forming a further miniaturized semiconductor device, heat treatment, which is performed when forming an insulative film on the inner surface of the STI trench or deep trench, may diffuse impurities from the N+ embedment layer 102 to positions close to the surface. This would deteriorate the transistor characteristics.
When the N+ embedment layer 102 is exposed from the deep trench, the impurities of the N+ embedment layer 102 is diffused outwards through the deep trench. This would lower the concentration of the impurities in the N+ embedment layer 102 and increase the collector resistance. Thus, when forming an insulative layer on the inner surface of the STI trench and on the inner surface of the deep trench, the prevention of defects resulting from the corners in the STI and deep trenches and the prevention of impurity diffusion from the N+ embedment layer 102 must both be performed at the same time.
In the element isolation structure of the prior art STI technique, the thicknesses of the insulative film on the inner surfaces of the STI trench and the deep trench are about the same. Thus, when preventing the occurrence of defects near the STI trench while rounding the corners of the deep trench, the heat treatment for forming the insulative layer on the inner surface of the deep trench increases the diffusion of impurities from the N+ embedment layer 102. When preventing the diffusion of impurities from the N+ embedment layer 102 during the formation of the insulative film on the inner surface of the deep trench, the corners, especially, those of the STI trench, cannot be sufficiently rounded. Thus, the occurrence of defects cannot be prevented. The corners of the STI trench are covered by the insulative film during the formation of an insulative film on the inner surface of the deep trench. Thus, it would be difficult for stress to be released from such corners. In such a case, stress is accumulated at the corners when forming the insulative film on the inner surface of the deep trench. This would increase the tendency of a defect occurring at a corner of the STI trench.